A PLL synthesizer system using a PLL loop, and a voltage synthesizer system using an AFT loop are both widely known as tuning systems.
When both systems are compared, the PLL synthesizer system has the advantages of higher speed, better accuracy and potential for stable reception, but also has a characteristic that its signal-to-noise ratio is inferior, due to harmonic wave components of a frequency divider signal in the PLL loop or a fluctuation of the loop. The voltage synthesizer system has the advantage of a higher S/N ratio, but has drawbacks in the tuning accuracy and stability, and a slow tuning velocity.
Therefore, there is a concept that the advantages of both systems are employed by combining both the PLL synthesizer system and the voltage synthesizer system. This concept is disclosed, for example, in Japanese patent application Laid-open Nos. 57324/1981, 52972/1981 and 48712/1981.
A liquid crystal television has been recently produced, commercially. Since this television employs a liquid crystal display unit having much less power consumption than a Braun tube, the television can be driven by a dry cell to enhance the portability. However, a reduction in the power consumption is still an important subject in the liquid crystal television, and the lifetime of the dry cell in the television is only several hours at present.
The conventional systems disclosed in the above-mentioned Japanese patent application Laid-open Nos. 57324/1981, 52972/1981 and 48712/1981 do not consider the use of liquid crystal television, nor the power consumption. The tuning device of the PLL synthesizer system has, in addition to the above-mentioned advantages, a disadvantage that the power consumption of a prescaler for dividing the local oscillation frequency of a tuner oscillating in a high frequency is large. This point is not as important in a television driven by a commercial power source, but when driven by a dry cell, as in a portable liquid crystal television, this becomes a very serious drawback. Therefore, the liquid crystal television cannot employ the PLL synthesizer system having the advantages of high speed and high accuracy.